ABSTRACT Nearly 80 million adults in the US currently suffer from hypertension, which is linked to endothelial dysfunction. The endothelium lines the lumenal surface of all blood vessels and has a primary role in controlling blood vessel tone and coordinating vasodilatory responses in resistance arteries. As such, endothelial dysfunction, is associated with the early stages of hypertension and in disease progression. In small resistance arteries, endothelial-derived vasodilation requires the release of calcium from intracellular stores i.e. the endoplasmic reticulum, to activate calcium-sensitive potassium channels. This induces hyperpolarization that spreads to adjacent smooth muscle cells, inducing vasodilation. A loss of control of intracellular calcium release in endothelial cells can impair blood pressure regulation and is a key therapeutic target in the treatment of hypertension. Pannexin proteins, which form large pore channels, have recently been shown to play vital roles in vascular biology and blood pressure regulation. Panx3 can localize to the endoplasmic reticulum where it has been shown to contribute to the release of stored calcium. While Panx3 has been largely unstudied in the cardiovascular system, our laboratory has published data illustrating its expression in endothelial cells. Our data show that Panx3 appears to be expressed in intracellular compartments, consistent with endoplasmic reticulum localization in endothelial cells. Preliminary data presented in this proposal, using a novel endothelial-specific Panx3 knockout mouse, demonstrate that resistance arteries lacking endothelial-Panx3 expression exhibit enhanced vasodilation upon stimulus as well as dysregulation of intracellular calcium activity in the endothelium at baseline and upon stimulus. Based on these data, I hypothesize that Panx3 functions as a passive calcium channel in the endoplasmic reticulum to regulate endothelial-mediated vasodilation. Aim 1 will demonstrate, for the first time, the precise subcellular localization of Panx3 in the endothelium in relation to the endoplasmic reticulum, golgi, mitochondria and plasma membrane. To conclusively distinguish between the various membranes of interest, I will visualize Panx3 expression on intact resistance arteries via comparative staining with super-resolution microscopy and immune electron microscopy. Aim 2 will visualize unitary calcium events in live endothelial cells of intact resistance arteries using high-speed confocal imaging to determine the source of calcium transported by Panx3 (extracellular vs stored calcium) and how the loss of Panx3 affects calcium reuptake. Aim 3 will test how endothelial expression of Panx3 contributes to vasodilation using pressure myography and the regulation of systemic blood pressure regulation. Overall, this proposal seeks to elucidate a role for Panx3 channels in endothelial cells. Identifying Panx3 as a functional regulator of intracellular calcium will highlight a potential for pharmacological targeting to enhance endothelial-derived vasodilation in patients suffering from endothelial dysfunction related hypertension.